Conventional high bypass ratio turbofan engines, which are included in the more general category of gas turbine engines and which may be used for aircraft propulsion, typically include a fan, booster, high pressure compressor, combustor, high pressure turbine, and low pressure turbine in serial axial flow relationship about a longitudinal centerline axis of the engine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan and booster via a second rotor shaft. The fan includes an annular disk and a plurality of radially extending blades mounted on the disk, wherein the disk and the blades are rotatable about the longitudinal centerline of the engine. Such fans are circumferentially enclosed by a containment casing which is specifically designed to be capable of containing a fan blade in the event that the fan blade is released from its disk during operation. In order to prevent or minimize the structural damage to the engine and aircraft should one or more fan blades be released from the disk due to a catastrophic failure of one or more blades, ingestion of debris, or other cause.
To reduce weight, prior art casings have attempted to minimize material thickness while meeting strength requirements. Therefore, reinforcing structures referred to as containment rings, comprising circumferential rings of increased thickness extending radially outward from the casing, have been used to provide the necessary strength in primary impact areas of the casing. The presence of these containment rings makes the casing sensitive to impact location. During a blade impact, the presence of the containment rings can cause localized areas of high strain leading to failures, and can also promote shear failures if a blade should impact the casing adjacent to the "hard spot" caused by the presence of a containment ring.
A example of a prior art containment system is disclosed in U.S. Pat. No. 4,417,848, issued to Dembeck et al. This patent discloses a containment system in which a fan casing shell 60 has a maximum thickness T.sub.max disposed in an axial location where the predicted likelihood of blade impact is greatest. The casing thickness is then tapered away from the maximum thickness section in order to save weight, either in continuous fashion or in discrete steps. Although this type of containment system does have a lower weight compared to a casing having a constant thickness, the Dembeck patent does not address the possibility of optimizing the use of material throughout the containment section.
Accordingly, it would be advantageous to provide an improved containment casing for turbomachinery, such as turbofan engines used in aerospace applications, wherein an impact resistant section is provided for the containment structure so as to enhance the ability of the containment structure to withstand an impact by a released fan blade and at the same time the containment structure should be relatively insensitive to the axial location of the impact of a blade released from a rotating disk while optimizing the thickness contour of material used for containment purposes.